Physical Layer Security Jamming: Theoretical Limits and Practical Designs in Wireless Networks

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by  Kanapathippillai Cumanan

AMANUSCRIPTACCEPTEDINTHEIEEEACCESS 1
Physical Layer Security Jamming: Theoretical Limits and
Practical Designs in Wireless Networks
Kanapathippillai Cumanan, Hong Xing, Peng Xu, Gan Zheng, Xuchu Dai, Arumugam Nallanathan, Zhiguo Ding
and George K. Karagiannidis
Abstract—Physicallayersecurityhasbeenrecentlyrecognized signals,whicharereferredasartificialnoise(AN)approachin
as a promising new design paradigm to provide security in the literature. On the other hand, a receiver can be also used
wireless networks. In addition to the existing conventional cryp-
7 to transmit jamming signals with the help of full duplex (FD)
tographicmethods,physical layer securityexploitsthedynamics
1 radios, which have the capability to simultaneously transmit
of fading channels to enhance secured wireless links. In this
0 approach, jamming plays a key role by generating noise signals andreceivethesignals.Hence,FDreceivercanbeexploitedto
2
toconfusethepotentialeavesdroppers,andsignificantlyimproves receive the required signals while sending jamming signals at

n quality and reliability of secure communications between le- thesametime toconfusetheeavesdroppers[6].However,this
a gitimate terminals. This article presents theoretical limits and transmit and receive jamming scheme might not be possible
J practical designs of jamming approaches for physical layer se-
0 curity.Inparticular,thetheoreticallimitsexploretheachievable under all circumstances due to limited available number of
2 secrecy rates of user cooperation based jamming whilst the antennasandthestrongself-interference(SI).Inthisscenario,
centralized, and game theoretic based precoding techniques are the externalnodes can be employed to send jamming signals,
] reviewedforpracticalimplementations.Inaddition,theemerging where they could be relay nodes or private jammers [10]–
T wirelessenergyharvestingtechniquesareexploitedtoharvestthe
[12]. Incase ofprivatejammers,theycouldintroducecharges
I required energy to transmit jamming signals. Future directions
fortheirdedicatedjammingservices.Theproblemsassociated
. of these approaches, and the associated research challenges are
s
c also briefly outlined. with these private jammers can be formulated into different
[ game theoretic problems by considering the legitimate nodes

and the private jammers as the players of the game.
1
I. INTRODUCTION This article focuses on physical layer security jamming
v
4 Inwirelesscommunications,theexponentialgrowthofmo- techniques based on user cooperations and external nodes.
6 biletrafficandnewlyemergingwirelessapplicationsintroduce Firstly,theoreticallimitsofjammingthroughusercooperation
7 different security risks due to their broadcasting nature. The ispresented,andthenmulti-antennabasedjammingtechniques
5
secured communication links in traditional wireless networks are reviewed by exploiting their spatial diversity and degrees
0
. are established through conventional cryptographic methods. of freedom (DoF). For example, the advantages of jamming
1 However, these methods impose different challenges in terms with multi-antenna transmitter can be easily demonstrated by
0
of key exchange and distribution, especially in the current appropriatelydesigningbeamformerssuchthatitwouldcause
7
1 trend of dynamic network configurations. Recently, physical a significant interference to the eavesdroppers while no or
: layer security has been recognised as one of the potential so- less interference leakage to the intended receivers. However,
v
lutionsto enhancesecurityin wireless networksby exploiting the study of theoretical limits of jamming and the practical
i
X characteristics of wireless channels [1]–[4]. In addition, this designs are necessary to achieve the optimal performance
r novel paradigm complements the conventional cryptographic in secrecy networks. This article presents these theoretical
a
methods, and well suits for dynamicnetworks and distributed limits and design approaches as follows. First, theoretical
processing techniques. limits of user cooperationbased jamming are explored. Then,
Physical layer security jamming is a well known approach centralized and game theoretic based multiple-inputmultiple-
toenhancethequalityofsecurewirelesstransmissions[5]–[9]. output(MIMO)transmitandreceiveprecodingtechniquesare
Inthistechnique,additionaljammingsignalsaretransmittedto discussed to provide efficient jamming services. In addition,
confusethepotentialeavesdroppersortodegradethedecoding wireless energy harvesting (WEH) based jamming techniques
capability of the unintendedreceivers.These jamming signals arepresentedthroughtherecentadvancementinsimultaneous
can be introduced by embedding them with the intended wireless information and power transfer (SWIPT) concept.
Finally, future research challenges of jamming schemes are
K. Cumanan is with the Department of Electronics, University of York, briefly discussed.
YO105DD,UK.
Z.DingiswiththeSchoolofComputingandCommunications, Lancaster
University Lancaster, LA14WA,UK. II. INFORMATION-THEORETICLIMITS OF JAMMING
H.XingandA.NallanathanarewiththeDepartmentofInformatics,King’s In this section, we present the theoretical limits of
College, London,UK.
cooperative-users based jamming for physical layer security.
P. XuandX. Dai are with the Department of Electronic Engineering and
InformationScience,UniversityofScienceandTechnologyofChina,China. The concept of cooperative jamming (CJ) was introduced in
G. Zheng is with the Wolfson School of Mechanical, Electrical and [5] for a Gaussian multiple access wiretap channel (GMAC-
Manufacturing Engineering, LoughboroughUniversity, UK.
WT) from an information-theoretic aspect. In the GMAC-
G. K. Karagiannidis is with the Department of Electrical & Computer
Engineering, Aristotle University ofThessaloniki, Thessaloniki, Greece. WT channel, multiple legitimate users wish to send secret
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W X 1 Y Wˆ 2
1 1 1 1
(cid:40)(cid:81)(cid:70)(cid:82)(cid:71)(cid:72)(cid:85) 1.8
1.6
a
(cid:36)(cid:79)(cid:76)(cid:70)(cid:72) b (cid:37)(cid:82)(cid:69) Secrecy Rates11..124 AS−based Scheme
X2 1 Y2 W1 Achievable 00..68
0.4
(cid:38)(cid:68)(cid:85)(cid:79)(cid:82) (cid:40)(cid:89)(cid:72) 0.2
(a) 0
1
S 2
1 3 IA Scheme
4
W1 (cid:40)(cid:81)(cid:70)(cid:82)(cid:71)(cid:72)(cid:85) X 1 X1 1 Y1 Wˆ1 a 5 6 7 6 5 4 b3 2 1 0
a Fig. 2: Achievable secrecy rates of the IA and AS-based
(cid:36)(cid:79)(cid:76)(cid:70)(cid:72) (cid:37)(cid:82)(cid:69) schemes for the WT-HI channel shown in Fig. 1 (a), where
the maximum transmit power at Alice and Carlo are 20 and
b
50, respectively. Since the IA scheme is a generalization of
the CJ andNFscheme,its achievablesecrecyrate is thesame
X2 1 Y2 W1 as that of the CJ scheme when b<1 and the same as that of
the NF scheme when b>1.
(cid:38)(cid:68)(cid:85)(cid:79)(cid:82) (cid:40)(cid:89)(cid:72)
(b)
Carlo can still help Alice and Bob using the noise forwarding
Fig.1:(a)Thewiretapchannelwitha helpinginterferer(WT- (NF) scheme in [14]. In NF, Carlo can randomly choose a
HI). (b) The WT-HI channel with artificial state. codewordfromaknowncodebookwithanappropriatecoding
rate such that the confusion signal can be decoded by Bob
before the decoding of W1, and hence, without affecting the
messages to a common receiver in the presence of a passive message-carryingsignal,stilljamstheeavesdroppingchannel.
eavesdropper. In order to maximize all the sum secrecy rate, An interpretation of NF is that the independent confusion
a user should send pure Gaussian noise as long as the eaves- codewords can bring additional randomness to the channel to
dropping channel from it to the eavesdropperis stronger than enhance the security level. The main difference between NF
the legitimate channel from it to the intended receiver. The and CJ is that, the former designs the confusion signals with
CJ scheme can be illustrated via a simple two-user GMAC- structure that does not jam Bob, whereas the latter uses pure
WT channel as shown in Fig. 1, where a transmitter (Alice) noise that jams Bob and Eve simultaneously.
wishes to send a secret message W1 to the intended receiver BothCJandNFcanbegeneralizedintoaunifiedframework
(Bob) under the help of an interferer (Carlo), without leaking based on the adaptive adjustment of the coding rate at Carlo,
any informationto the eavesdropper(Eve). The channelgains which is called interference assisted (IA) scheme in [13] for
fromAlicetoBobandEvearenormalizedtobe1and√a;the the WT-HI channel. The main difference between the IA and
channelgainsfromCarlotoBobandEvearenormalizedtobe NF schemes is that, in the former, Carlo treats its coding
√b and 1. Note that the channelgainsfrom Alice to Bob and rate as a variable, and adaptively adjusts this coding rate
fromCarlo to Eveare differentwith each other althoughboth to maximize the achievable secrecy rate; while in the latter,
ofthemarenormalizedto1,whichisreflectedbythefactthat Carlo always chooses a coding rate such that the interference
boththechannelshavedifferenteffectsonthereceivedsignal- transmitted by him does not affect Bob. Both CJ and NF can
noise-ratios (SNRs) at Bob and Eve. This channel model is be viewed as the special cases of the IA scheme. When the
also called the wiretap channelwith a helpinginterferer(WT- coding rate at Carlo is lower than a certain rate such that
HI) in [13]. When b < 1, Carlo can help Alice and Bob Bob can decode the interference before decoding the secret
to enhance the security level by sending Gaussian noise that message, the IA scheme reduces to NF; when the coding rate
is independent of the message-carrying signal. This can be at Carlo is sufficiently large such that both Bob and Eve have
interpretedby the factthat Carlo’s jamming signalharmsEve no choice but to treat the interference as pure noise, the IA
morethanBobwhenb<1,whichmayimprovetheachievable scheme reduces to CJ.
secrecy rate. CJandNFcanalsobegeneralizedbythechannelprefixing
CJ is usefulonly when b<1 in the WT-HI channelin Fig. scheme in secure communications.As shown in the Gaussian
1 (a). But, when b > 1, i.e., Carlo has a stronger channel to WT-HI channelin Fig. 1 (a), the confusion signal transmitted
Bob than to Eve, CJ in [5] might not be useful. In this case, by Carlo can consist of two parts, i.e., X2 =U2+Z2, where
3
onepart(U2)isthecodewordwiththestructurefollowingthe III. MIMO JAMMING:CENTRALIZED APPROACH
NF scheme, and the other part (Z2) is pure Gaussian noise In this section, we review key results of using MIMO to
following the CJ scheme. In this channel prefixing scheme,
transmit judiciously jamming signals with an aim to achieve
the channel input X2 at Carlo becomes a noisy version of higher secrecy rate. Specifically, centralized transmit and
the interference codeword U2 in terms of the Markov chain receive jamming techniques are discussed in the following
U2 X2 Y1,Y2.InadditiontoCarlo’schannelprefixing,the subsections.
− −
workin[15]alsoadoptedachannelprefixingschemeatAlice
based on artificial state (AS), and the achievable secrecy rate
A. Transmit Jamming
can be further enhanced. Specifically, Alice spends part of its
Consider a basic three-node system, which consists of a
transmit power to generate an AS S1, i.e., it sets the channel
input as X1 = X˜1 + S1 as shown in Fig. 1 (b). Now, the transmitter, an intended receiver and an eavesdropper. The
transmitter has multiple antennas, while the receiver and the
WT-HI channel in Fig. 1 (a) becomes a WT-HI channel with
eavesdroppermay havemultiple antennasor a single antenna.
state information as shown in Fig. 1 (b), where the random
state S1 is known by Alice a priori and X˜1 can be viewed as The secrecy capacity is well known [1] when the channel
state information (CSI) is available to all nodes. While it is
the virtual channel input. Then following the concept of dirty
reasonable to assume to have the receiver’s CSI, it is usually
paper coding (DPC), we introduce an auxiliary variable U1 =
X˜1 +βS1 as the message-carrying codeword. According to unrealistictoobtaintheeavesdropper’sCSI.Inthecasewhere
only the receiver’s CSI is known but the eavesdropper’s CSI
DPC, S1 will notaffect the decodingof the message-carrying
is absent, the transmit jamming or AN is an effective means
codeword U1 at Bob by appropriately setting the value of β.
to improve the secrecy rate. As shown in Fig. 3 (a), the
Fig. 2 shows achievable secrecy rates of the IA and AS- total transmit signal is split into two parts. The first one is
based schemes for the WT-HI channel shown in Fig. 1 (a), the information-bearing message towards the direction of the
where the maximum normalized transmit power at Alice and receiver, which can be designed based on the receiver’s CSI.
Carlo are 20 and 50. For both schemes, Gaussian codebooks The second part is the transmit jamming, which is isotropic
are adopted,where the message-baredsignal X1 (0,P1), Gaussian noise in the orthogonal space of the receiver’s
∼N
and the Gaussian interference X2 (0,P2), 0 P1 20, channel.Bydoingso,thetransmitjammingdoesnotaffectthe
0 P2 50. Furthermore, the AS∼-bNased scheme≤sets X≤˜1 intended receiver but only degrades the quality of the signal
≤ ≤ ∼
(0,(1 λ)P1), S1 (0,λP1). The optimal values of these received by the eavesdropper. A remarkable result about this
− ∼
parameters(P1,P2,λ)areestablishedviaexhaustivesearches. simple transmit jamming scheme shown in [1] is that, it can
As shownin Fig. 2, whena>1, the ASatAlice isimportant achieve secrecy rate close to the capacity in the high SNR
to get a larger secrecy rate if compared with the NF (or IA) regime when the receiver has a single antenna.
scheme. Particularly, when b = 1, Carlo can still assist Alice The use of transmit jamming consumes some transmit
to achieve a positive secrecy rate, whereasthe CJ, NF and IA power so it reduces the SNR at the receiver. Therefore it
schemes fail. This can be interpreted by the fact that the AS is important to allocate power to the information-bearing
atAlicecanassociatewiththehelpinginterferenceatCarloto signals and the jamming signals properly. While it is in
further confuse Eve without affecting Bob. Fig. 2 also shows general difficult to achieve the optimal power allocation, a
that the gap between the AS-based and the NF schemes can general rule of thumb is to allocate more power to jamming
be enlarged by increasing a from 1 to 7. Moreover, the AS- when the receiver’s CSI is more accurate or the number
basedschemereducestotheCJ(orIA)schemewhenb<1/a, of the eavesdropper’s antennas increases to achieve effective
thanks to the channel prefixing at Carlo. jamming.
The works in [5], [13]–[15] used information-theoretical
Gaussian codebooks with infinite alphabets, which is hard B. Receive Jamming
to implement in practice. Alternatively, the work in [16] has Transmit jamming is useful but requires the support of
proposedanachievableschemebasedonlayerednestedlattice multiple antennas. When the transmitter has a single antenna,
codes with an finite alphabets. Interestingly,unlike traditional is it still possible to take advantage of jamming? Fortunately,
communications without secrecy constraints, the use of more there is a positive answer to this question. One possible
practical lattice codes can outperform the Gaussian code- solution is to use jamming at the receiver side to confuse
book for the channel model in Fig. 1 (a) for certain cases. the eavesdropper. There are different implementations. For
Specifically, the scheme in [16] can achieve non-zero secure instance, the transmitter repeats the transmission for a certain
degreeoffreedom(s.d.o.f.)forall valuesof channelgainpair times, and the receiver randomly jams the transmissions [17].
(√a,√b) except when ab = 1, whereas the works in [5], Because the eavesdropper does not know which transmission
[13]–[15] fail to achieve non-zero s.d.o.f.. This is because, isleftunjammed,itcannotdecodethemessagecorrectly.One
based on nested lattice codes, the signals transmitted by both drawback of this scheme is that, it requires retransmission,
the source and the cooperative jammer can be aligned at the whichislessbandwidthefficientandmaybecriticalfordelay-
eavesdropper but remain separable at the intended receiver; sensitive applications. Next we introduce a receive jamming
whereas the Gaussian noise transmitted by the cooperative scheme without using retransmissions.
jammer simultaneously interferes with the eavesdropper, and Traditionallyawirelessnodeworksinthehalf-duplex(HD)
hurts the intended receive. mode. The FD operation, which allows a wireless node to
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(a) Transmit Jamming (b) Receive Jamming
... ... ... ...
TX RX
Eavesdropper Receiver Eavesdropper Full-duplex
Receiver
Information
... bearing signal
Jamming
signal
Self-interfernce
Transmitter Transmitter
Fig. 3: MIMO Jamming. (a) Transmit jamming when the transmitter has multiple antennas; (b) Receive jamming when the
receiver has multiple antennas and works in the FD mode.
transmit and receive simultaneously in the same frequency Player 1 Player 2
band has emerged as an attractive solution to improve the Interference Power (p) Interference Price (µ)
&#$ !%#$ !"#$
spectral efficiency. The FD operation has also shown great
potential to improve physical layer security. As depicted in
Fig. 3(b), the main idea is that the intended receiver sends
jamming signals to degrade the eavesdropper’s channel and ’()(*+($
protect its own reception [6]. Obviously the receive jamming
’()(*+($
will affect both the intended receiver itself and the eaves-
dropper because of the resulting SI, but if the SI can be
well controlled/optimized, it will favor the intended receiver.
When the receiver has multiple transmit or receive antennas, Transmitter Jammers
it can employ joint transmit and receive beamforming for (a)
simultaneous signal detection, SI suppression and jamming
emission. An interesting result in [6] is that when the global Fig.4:(a)AMIMOsecrecynetworkwithmultipleeavesdrop-
CSI is available, the secrecy rate increases unbounded as the pers and private jammers. (b) The concept of equilibrium for
SNR goes up, which is in contrast to the traditional HD case the transmitter-private jammers game.
withoutreceive jamming.Even when only statistical informa-
tion about the eavesdropper’s channel is known, substantial
secrecy rate improvement is observed. Obviously, transmit
conditions and the strong SI. In this scenario, external
and receive jamming can be combined to further improve the
jammers can be employed to improve the quality of the
secrecy rate in a MIMO wiretap channel. In addition, both
secure communications by introducing jamming signals to
of these jamming techniques can be directly applied to the
the eavesdroppersas shown in Fig. 4 (a) [10]–[12]. However,
relay jamming schemes, where a set of trusted half-duplex
these external (private) jammers charge for their dedicated
or full-duplex relays can help the communication between
jamming services from the transmitter based on the amount
legitimate terminals while introducingjamming signals to the
of interference caused at the eavesdroppers. In order to
eavesdroppers.Thehalf-duplexrelayscouldintroduceartificial
compensate these prices, the transmitter introduces charges
noise to the eavesdroppers while transmitting the signal to
for its enhanced secure transmission from the legitimate
the legitimate users. On the other hand, the full-duplexrelays
users. In this scenario, both the transmitter and the private
could transmit jamming signal, while receiving the signals
jammers compete to maximize their revenues by providing
fromthesource.Specifically,theartificialnoiseapproachwith
higher secrecy rates at the legitimate users and selling the
half-duplexrelayscanbe consideredasthe transmitjamming,
interference to the transmitter, respectively.
whereas the receive jamming represents the full-duplex relay
In order to analyze the interactions between the
based jamming scheme.
transmitter and the private jammers, game theory provides
the mathematical structure and concepts to formulate this
IV. MIMOJAMMING: GAME THEORETICAPPROACH scenario into different games, where both the transmitter
In this section, we review game theoretic based jamming and the private jammers will be the players of the game
approaches for physical layer security. The centralized [18]. In addition, analytical results obtained through these
transmitter-receiver based jamming schemes discussed in games based on the strategic decisions of the players will
the previous section might not be able to achieve the help to conclude whether there is an equilibrium in their
required performance under all circumstances due to channel revenues for the proposed game as shown in Fig. 4(b), at
5
which both players achieve their maximum revenues. On approach, where the bids from the transmitters and power
the other hand, achieving these equilibria among the players allocations between the users will be updated iteratively.
mightintroducemorecomplexityin thenetwork,eventhough
they are beneficial for all players. To circumvent these
V. ENHANCING PHYSICAL LAYERSECURITY IN WIRELESS
complexity issues, game theory also facilitates to develop
ENERGYHARVESTING NETWORKS
distributedandlowcomplexitybasedimplementationtoattain
these equilibria. Based on whether it is a collaboration or Since there has been an upsurge of research interest as
competition among the players, these games can be classified wellasemergingapplicationsforradio-frequency(RF)signal-
into cooperative and non-cooperative games, respectively enabledwirelessenergytransfer,whichavailsthebroadcasting
[18]. andfar-fieldradiativepropertiesofelectromagnetic(EM)wave
Asecrecynetworkmodelisconsideredwithonetransmitter to power wireless devices in particular, while transferring
and one legitimate user, where private jammers help to information, new challenges as well as opportunities for
improve the secure transmission by causing interference to physical layer security begin arising in these WEH-enabled
the eavesdroppers as shown in Fig. 4(a). The transmitter, the networks.Inthis section, we discussthe-state-of-arttechnolo-
legitimate user and the private jammers are all equipped with gies enhancingphysicallayer security for one importantclass
multiple antennas. As mentioned earlier, the interaction of of WEH application, i.e., wireless powered communication
buying interference from private jammers and announcing network (WPCN).
interference prices to the transmitter can be formulated Physical layer security issues in the rapidly developed
into a Stackelberg game, where the private jammers and cooperative networks such as device-to-device systems, relay
the transmitter are the leaders (Player 1) and the follower networks etc., have already drawn significant attention and
(Player 2) of the game, respectively [18]. In order to study inter alia, CJ has been widely studied as a promising tech-
this game, the best responses of both the players should be nology [5], [7]. However, the benefits of CJ would be quite
derived, where the best interference requirements for a given compromised if those energy-limited potential helpers are
interference price and the best interference price for a given unwillingtocooperate.Inthefollowing,weintroducehowthis
interferencerequirementswillbeobtainedatboththe players. bottleneck could be broken with self-sustainable terminals in
Basedonthesebestresponses,theStackelbergequilibriumcan a WPCN. Benefiting from dense radio-frequency(RF) signals
be achieved by deriving the optimal interference requirement of increasing amount of data transmission in the cooperative
and the interference prices. At this equilibrium as shown in networks, a newly designed two-phase protocol, i.e., harvest-
Fig. 4(b), both of the players will achieve their maximum and-jam (HJ) was proposed in [19] to achieve secrecy trans-
revenuesandthedeviationofanyplayerfromthisequilibrium missionbyCJandyetnottoaddextrapowercost.Specifically,
will cause a loss in their revenues. This equilibrium can be asillustratedinFig.5,inthefirsttransmissionphase,asingle-
implemented by exchanging the associated channel responses antenna transmitter sends confidential information to a multi-
betweenthetransmitterandtheprivatejammers.However,this antenna amplify-and-forward (AF) relay with conventional
might introduce more complexity in the network. Therefore, power supply and simultaneously transfers power to a group
a distributed implementation of this equilibrium would of idle multi-antenna users serving as helpers; in the second
be more appropriate to reduce the complexity, where the transmission phase, the AF relay amplifies, and forwards the
interferencepricesofferedbythejammersandtheinterference message to the legitimate receiver under the protection of
requirementsatthetransmittercanbeiterativelyupdated[10]. jamming, which is generated from each of the helpers by
its harvested power in the previous transmission phase. The
The secrecy network might consist of multiple legitimate secrecy rate is maximized by optimizing the transmit beam-
multi-antenna transmitter-receiver pairs as well as a friendly forming matrix for the AF relay and the jamming covariance
jammer in the presence of multiple eavesdroppers. In this matrices for each of the helpers subject to transmit power
scenario, each pair tries to maximize its secrecy rate with constraints, undercircumstancesof perfectand imperfectCSI
the help of the friendlyjammerby introducinginterferenceto available at the coordinating node, respectively. The results
the eavesdroppers. However, the jamming signals will cause in [19] showed that the HJ scheme plays a prominent role
interference not only to the eavesdroppers but also to the in improving secret communications in practical scenarios of
destinations,whichwilldegradethesecrecyrateperformance. imperfectCSI.Especially,whenCSIsrelatedtotheeavesdrop-
Therefore, the jammer should distribute its power among the per are hard to obtain and more imperfect than those related
users such that the secrecy rate performance is improved. to the legitimate users, the optimal power allocation scheme
These interactions between the pairs and the jammer can be inclinestojamtheeavesdropperusingallavailablepower,and
formulated into an auction game, where the transmitters and hence considerably degrades its information reception while
the jammer will be the bidders and auctioneer [18], respec- minimizing the interference caused to the legitimate receiver.
tively. The transmitters will submit their bids to the jammer Typically, in a WPCN setup where helpers are fixed and
dependingonthepaymentofthecorrespondingjammer-power evenly distributed around a disk centered on the transmitter
and the secrecy rate improvement whereas the jammer will with radius 2m , assuming a simple channel fading model
determine the optimal power allocations between the users comprising Rayleigh fading and pathloss given by ( d )−α,
d0
based on these bids. For this game, a distributed solution can given the reference distance d0 = 1m and the attenuation
be developed by exploiting the distributed auction theoretic factor α=3, if the transmit power is set to be 0dBm and the
6 AMANUSCRIPTACCEPTEDINTHEIEEEACCESS
Future work of interest is to design more intelligent coding
First transmission phase Energy Transfer schemesto achievethe secrecy capacity.For example,though
Second transmission phase Jamming theAS-basedschemein[15]showedthatthechannelprefixing
technique is crucial to improve the achievable secrecy rate,
the effects of channel prefixing has not been fully revealed.
Moreover, this paper only considers the jamming schemes
based on Gaussian random coding, which cannot achieve a
positive secure DoF at high SNR, while recent works have
shownthatthenon-Gaussiancodes(e.g.,structuredcodes)can
achieve a positive secure DoF at high SNR. Hence the design
of non-Gaussian codes is also an interesting future research
area.
B. Smart MIMO jamming
Fig. 5: A WPCN model employing harvest-and-jam (HJ)
protocol. ThemajorityofliteratureassumesidealGaussiansignalling
and therefore the optimal jamming also uses Gaussian sig-
nalling. However, Gaussian signalling is not practical and
energy conversion efficiency is 50%, the average per-antenna wireless systems often employ constant envelop signalling
harvested power at each helper is around 62.5µW. schemeslikephase-shift-keying(PSK).Inthiscase,itisshown
Besides increasing the secrecy rate (short-term metric) via that Gaussian jamming is no longer effective, and can be
adaptive power allocation over antennas for AF beamform- removedbythemulti-antennaeavesdropperusingblindsource
ing and jamming signal design, [20] considered a similar separation techniques such as constant modulus algorithms
idea of increasing the confidential information transmission [21]. Therefore, there is a need to design the smart jamming
throughput(long-termmetric)via awireless-poweredfriendly signals adaptive to the specific constellation used.
jammer by proposing a delicately designed protocol with not MassiveMIMOisagreatenablertoachievespectrum-efficient
necessarily equal time duration for the “harvest” and “jam” and energy-efficient wireless communications. However, its
phases. Specifically, defining a “power transfer (PT)” block securityimplicationisnotwellunderstood.Ourpreviouswork
for the jammer to harvest energy from the source and an shows that massive MIMO systems, if carefully designed, are
“information transmission (IT)” block for confidential infor- actually quite robust against both passive and active attacks
mation to be transmitted under the protection of a jamming [22].Furtherinvestigationisneededtounderstanditspotential
signal,athresholdamountofenergy,i.e., JT,whichsupports to effectively jam the eavesdroppers.
P
jamming using power J for a transmission block of T
P
time unit, is examined at the jammer’s battery storage at C. Robust jamming games
the beginning of each transmission block. Only when this
EffectivejammingrequiresperfectCSI,whichisdifficultto
threshold is achieved, and meanwhile the source-destination
obtaininpractice.ImperfectCSImaynotonlydegradetheper-
(main channel) does not suffer from communications outage,
formanceof the legitimate communicationsbut also results in
the IT blockwith jammingstarts. Otherwise,the transmission
informationleakagetotheeavesdroppers.Inordertodealwith
enters into either dedicated PT block (when the threshold
this issue, it is necessary to consider robust transmit jamming
condition is not satisfied) or opportunistic PT block (when
to achieve guaranteedoutage performanceof the secrecy rate.
communication outage over the main channel occurs). Based
Most of the game theoretic approaches proposed for physical
on this protocoldesign, four types of PT-IT cycles consisting
layer security have been assumed that the players have the
of varied combination of these blocks are characterized, and
perfect CSI of the eavesdroppers. In order to overcome the
as a result, the long-term behavior of this stochastic process
imperfect CSI issues associated with eavesdroppers in the
has been analyzed with a closed-form achievable throughput.
existing games, robust techniques should be considered for
Finally, the design parameters, J and the fixed transmission secure communications.Therefore,the developmentof robust
P
ratesareoptimizedtofurthermaximizethesecrecythroughput
jamming games by incorporating channel uncertainties or the
under the constraint of the secrecy outage probability.
cases of no eavesdroppers’ CSI along with the corresponding
analysisofequilibriawouldbeverychallenging.Theserobust
VI. RESEARCH CHALLENGES jamming games could be formulated into Bayesian games,
In this section, we provide future research challenges as- which are well known for the scenarios with incomplete in-
sociated with the physical layer security jamming schemes formation. The jamming games with imperfect eavesdroppers
discussed in the previous sections. CSI would be one of the possible interesting futuredirections
in game theoretic based jamming for physical layer security.
A. Theoretical limits
Thoughexistingworkshaveproposeda varietyofjamming D. Wireless energy harvesting
schemes to enhance security level, the secrecy capacity has More sophisticated transmission protocols incorporating
not been found even for the simple WT-HI channel in Fig. 1. adaptivepower/ratedesignsareexpectedtobestudiedforfur-
7
ther improving the long-term secrecy throughputin [20]. Be- [14] L.Lai,andH.ElGamal,“Therelay–eavesdropperchannel:Cooperation
sides, to further motivate the potential WEH-enabled helpers forsecrecy,”IEEETrans.Inf.Theory,vol.54,no.9,pp.4005–4019,Sept.,
2008.
to assist secure communications, more practical energy and
[15] P. Xu, Z. Ding, X. Dai and K. K. Leung, “A general framework of
communications mechanism needs to be properly designed. wiretap channel with helping interference and state information,” IEEE
For example, neighbour-users of the transmitter could be Trans.Inf.ForensicsSecurity, vol.9,no.2,pp.182–195,Feb.,2014.
[16] X. He and A. Yener, “Providing secrecy with structured codes: Two-
self-interested and prefer to storing the harvested energy for
user Gaussian channels,” IEEE Trans. Inf. Theory, vol. 60, no. 4, pp.
their own use rather than help jam. In this situation, the 2121–2138,Apr.2014.
transmitterneedstooffersomesparecommunicationsresource [17] S.GollakotaandD.Katabi,“Physicallayerwirelesssecuritymadefast
and channel independent,” in Proc. IEEE Int. Conf. Comp. Commun.,
as incentives, such as spectrum, to increase the cooperation
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utility and reduce the overall system cost. [18] Z. Han, D. Niyato, W. Saad, T. Basar and A. Hjørungnes, “Game
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VII. CONCLUSIONS [19] H.Xing,K.-K.Wong,Z.ChuandA.Nallanathan,“Toharvestandjam:
In this article, the jamming techniques for physical layer aparadigm ofself-sustaining friendly jammers forsecure AFrelaying,”
IEEETrans.SignalProcess.,vol.63,no.24,pp.6616–6631,Dec.2015.
security have been discussed with different approaches. In
[20] W.Liu, X.Zhou, S.Durrani and P.Popovski, “Secure communication
particular, the theoretical limits of user cooperation based with a wireless-powered friendly jammer,” IEEE Trans. Wireless Com-
jamming and the practical designs of MIMO jamming as mun.,vol.15,no.1,pp.401–415,Jan.2016.
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well as the game theoretic based jamming techniques have
multi-antenna wireless networks, ”IEEEInt. Symp. Information Theory
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also been presented for improvingenergyefficiency in secure [22] D. Kapetanovic, G. Zheng and F. Rusek,“Physical layer seucrity for
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generatejammingsignalsto the eavesdroppers.Finally,future
research challenges of these jamming schemes for physical
layer security have been briefly outlined.
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